Means for controlling an electron beam



Aug. 3, 1965 R. B. LOCHINGER 3,199,039

MEANS FOR CONTROLLING AN ELECTRON BEAM Filed Dec. 26, 1962 FIG.

C6"8 INVENTOR.

United States Patent 3,199,039 MEANS FOR CONTIIEOZIQLING AN ELECTRON Rolf B. Lochinger, Middlesex County, N.J., assignor to Radio Corporation of America, a corporation of Delaware Filed Dec. 26, 1962, Ser. No. 247,798 4 Claims. (Cl. 328-258) This invention relates generally to means for controlling an electron beam, and more particularly to improved bias means for varying the intensity of an electron beam intermittently. The improved bias means of the present invention are particularly suitable for operating an electron beam in electron beam machining apparatus.

In performing a machining operation on a workpiece by an electron beam in electron beam machining apparatus, it is usually desirable to pulse the electron beam, that is, to operate the electron gun intermittently. Such pulsed operation of the electron gun tends to prolong its life and to prevent overheating of the workpiece. The electron gun should be pulsed preferably with substantially square-wave pulses in order to prevent defocusing of the electron beam at the workpiece. It has been proposed to bias the electron gun with a bias resistor across which square-wave pulses are applied. Since the bias voltage for this purpose is ordinarily relatively large, in the neighborhood of several hundred volts, the required bias resistors may either dissipate too much energy or provide too long a time constant with the interelectrode capacity of the electrodes of the electron gun. Such long time constants increase the rise and decay times of the applied bias voltage, distorting the desired square-wave bias voltage. Pulse transformers have also been proposed for pulsing an electron gun, but pulse transformers do not provide the wide range of square-wave pulse widths that are usually required in electron beam machining apparatus.

It is an object of the present invention to provide improved means for biasing an electron gun with substantially square-wave voltage pulses whose pulse widths may be varied over a relatively wide range.

Another object of the present invention is to provide improved bias means for operating an electron gun intermittently in a manner suitable for performing a machining operation on a workpiece in electron beam machining apparatus.

A further object of the present invention is to provide an improved circuit for biasing an electron gun with substantially square-Wave voltages whose Waveforms have relatively short rise and decay times.

Still a further object of the present invention is to provide improved bias means of the type described that are relatively simple in circuitry, easy to operate, and highly efiicient in use.

Briefly, the improved bias means of the present invention comprise a variable impedance device connected in the bias circuit of an electron gun, and means to vary the impedance of the variable impedance device sharply with a substantially square-wave voltage. In a preferred embodiment of the present invention, the variable impedance device comprises a vacuum tube whose anodecathode impedance forms part of the bias circuit. The cathode and the grid of the tube are connected to the output of a flip-flop circuit whose squarewave output frequency and pulse width are determined by the frequency of input trigger pulses. By controlling the conduction, and thereby controlling the impedance of the tube, with the square-Wave output from the flip-flop circuit, the bias voltage of the electron gun is varied sharply. When the tube is rendered conductive, its impedance forms a relatively very short time constant with the interelectrode 3,199,039 Patented Aug. 3, 1965 capacity of the electron gun, resulting in the application of a square-wave bias pulse with a relatively very fast rise time. When the triode tube is rendered substantially nonconductive, the output tube of the flip-flop circuit is conducting heavily, and the relatively low impedance of the latter forms a relatively short time constant with the interelectrode capacity of the electron gun, resulting in a relatively very short decay time for the square-wave bias pulse.

The novel features of the present invention, both as to its organization and method of operation, as well as additional objects and advantages thereof, will be more readily understoodfrom the following description, when read in connection with the accompanying drawing, in which:

FIG. 1 is a schematic diagram of a circuit embodying the present invention;

FIG. 2 is a graph of trigger pulses suitable for determining the frequency and pulse width of square-wave bias voltages in the circuit of FIG. 1; and

FIG. 3 is a graph of the square-wave bias voltages of the type applied to an electron gun in the embodiment of FIG. 1.

Referring, now, to FIG. 1 of the drawing, there are shown means for biasing intermittently an electron gun 10 of the type used in electron beam machining apparatus. The electron gun 10 comprises a cathode K and a grid G, the interelectrode capacity between the cathode K and the grid G being represented by a capacitor C (in dashed outline). The remainder of the electrodes of the electron gun 10, such as the anode, for example, are not shown as these are known.

The bias circuit for the electron gun 10 comprises a potentiometer R and a variable impedance device, such as a triode tube TB. The variable tap of the potentiometer R is connected to the cathode K, the resistor of the potentiometer R being connected in series with the anode-cathode path of the tube TB and the grid G. A voltage is applied across the potentiometer R by a power supply 12.

The power supply 12 comprises a transformer 14 for transforming any suitable source of A-C voltage, applied across its primary winding 16, to a desired A-C voltage across its secondary winding 18. The voltage across the secondary winding 18 is rectified by a diode 20, and the rectified voltage is applied across the potentiometer R. A filter capacitor 22 is connected across the potentiometer R, and a pair of filter capacitors 24 and 26 are connected from the tap of the potentiometer R to op posite ends, respectively, of the resistor of the potentiometer R. Since the electron gun 10 is adapted to operate at relatively high voltages, in the neighborhood of kilovolts, for example, the primary winding 16 is electrostatically shielded from the secondary winding 18 by a grounded shield S, the secondary winding 18 also being insulated from the primary winding 16 to withstand very high voltages.

To vary the intensity of the electron beam from the electron gun 10, the bias voltage on the electron gun 10 is varied periodically by changing the impedance of the variable impedance tube TB periodically. This is accomplished with the aid of a bistable multivibrator 28 which comprises triodes T1 and T2 whose grids and anodes are cross-coupled through resistors 31) and 32, respectively, in a known manner. The cathodes of the tubes T1 and T2 are connected to each other and to a lead 34 through a common cathode resistor 36, the latter being bypassed by a capacitor 38. The grids of the tubes T1 and T2 are connected to the lead 34 through grid resistors 40 and 42, respectively.

An operating voltage for the multivibrator 28 is obtained by rectifying a portion of the A.-C. voltage across the secondary winding 18 of the transformer 14. A tap on the secondary winding 18 is connected to the cathode of a diode 44, and the anode of the diode 44 is connected to the lead 34. The anode of the diode 44 is also connected to an end of the secondary winding 18 through a capacitor 46 for filtering purposes. The common junction of the anode of the tube TB and the potentiometer R is connected to the last-mentioned end of the secondary winding 18 through a lead 48. It will be observed that the lead 48 is positive with respect to the lead 34 because of the direction in which the diode 44 is poled. The anode of tube T2 is connected to the lead 48 through a load resistor 50, and the anode of the tube T1 is connected to the cathode of the tube TB through a load resistor 52. The anode of the tube T1 is also connected to the grid of the tube TB to vary the impedance of the tube TB in accordance with variations of the voltage at the anode of the tube T1.

The output from the multivibrator 28, at the anode of the tube T1, is substantially square-wave pulses whose frequency and pulse width are determined by the frequency of trigger pulses applied to the input of the multivibrator 28, the input being the grid of the tube T2. Trigger pulses from any suitable pulse source P are applied through a transformer 54 and a coupling capacitor 56 to the input of the multivibrator 28. One end of the secondary winding 58 of the transformer 54 is connected to the grid of the tube T2 through the capacitor 56, and the other end of the secondary winding 58 is connected to the lead 34. The secondary winding 58 is insulated, by physical separation, from the primary winding 60 for very high voltages.

Typical triggering pulses for triggering the multi-vibrator 28 are illustrated in FIG. 2. These pulses, such as pulses P1 and P2, occurring at times t1 and t2, are the difierentiated pulses from the pulse source P, which may be a square-wave generator. Thus, for example, transformed pulses from the pulse generator P are differentiated by the differentiating network formed by the capacitor 56 and the resistor 42 and provide a series of positive and negative pulses P1 and P2 periodically. It is also within the contemplation of the present invention for the multivibrator 28 to be of the monostable type that may be triggered by pulses of one polarity.

In operation, the tubes of the multivibrator 28 are rendered conductive alternately, that is, one tube is conducting while the other tube is biased to a nonconducting state. Let it be assumed, for example, that the tube T1 conducts initially. Since the grid of the tube TB is connected to the anode of the tube T1, the grid of the tube TB is in its most negative state when the tube T1 is conducting. Under these conditions, the impedance of the tube TB is relatively very high, and the tube TB conducts very little current, so that it may be said to be virtually cut off. When the tube TB is conducting, its impedance is relatively very low and the voltage across the tube TB is also relatively low, so that the bias voltage of the electron gun is substantially the voltage across the portion R1 of the potentiometer resistor R between the tap and the anode of the tube TB. The tap on the potentiometer R is adjusted so that the voltage across the portion R1 is the bias voltage of the electron gun 10 for producing a beam of electrons of desired intensity from the electron gun 10.

The method of operating the electron gun 10 intermittently by varying its bias voltage with substantially squarewave pulses will now be described. Let it be assumed that, at time t1, the pulse P1 is applied to the grid of the tube T2. The tube T2 is rendered conductive, and the tube T1 is rendered nonconductive. Under these conditions, the voltage at the grid of the tube TB goes positive, and the tube TB conducts heavily. The bias voltage across the electron gun 10 is now substantially the voltage between the tap on the potentiometer resistor R and the anode of the tube TB, that is, the voltage across the resistor portion R1. The electron gun 10 is now biased to produce an intense beam of electrons capable of performing a machining operation on a workpiece (not shown).

At time t2, the negative-going pulse P2 is applied to the grid of the tube T2 and causes the tube T2 to cease conducting. The tube T1 now becomes conductive, and the voltage at its anode becomes less positive with respect to its cathode. Under these conditions, the grid of the tube TB goes negative with respect to its cathode and causes current through the tube TB to be virtually cut off. The tube TB now represents a relatively high impedance and the voltage thereacross, together with the voltage across the portion R1, are applied in series aiding relation between the cathode K and the grid G of the electron gun 10 to increase the bias therebetween. The electron gun 10 now ceases to produce an electron beam. An intense electron beam can be initiated once more by a positive-going pulse P1.

Referring, now, to FIG. 3 of the drawing, there are shown voltage waveforms occurring during the operation of the electron gun 10. The dashed line 64 represents the cut-off voltage of the electron gun 10. When the pulse P1 is applied to the input of the multivibrator 28, at time t1, the voltage at the grid G of the electron gun 10 rises sharply, as shown by waveform portion 66. At time 12, when the pulse P2 is applied to the input of the multivibrator 28 and the tube TB is virtually cut off, the voltage at the grid G goes negative, as illustrated by waveform portion 68. The rise time and decay time of each pulse of the biasing voltage for the electron gun 10 should be relatively very short, as illustrated by the rise and decay waveform portions and 72, respectively. Unless this is so, the cross-sectional area, including the focus, of the electron beam emitted from the electron gun 10 tends to vary and render the electron beam unsuitable for machining purposes. The rise time (as shown by the Waveform portion 70) of the bias voltage is relatively very short because the interelectrode capacitance of the electron gun 10, represented by the capacitor C, and the relatively low impedance of the tube TB during its heavily conductive state form a relatively short time constant for the discharge path of the capacitor C. The relatively very fast decay time (as shown by the waveform portion 72) of the bias voltage is provided by the relatively short time constant of the capacitance of the capacitor C, the resistance of resistor 52, and the low impedance of the conducting tube T1, furnishing a fast charging path for the capacitor C. It is the relatively rapid charging and discharging of the interelectrode capacitor C that cause the applied bias voltages to the electron gun to be substantially undistorted squarewave pulses. Such operation of the electron gun produces a pulsed electron beam whose focus remains substantially unchanged during each pulse.

From the foregoing description, it will be apparent that there has been provided improved means for varying the bias of an electron gun for controlling the electron discharge intermittently. While only one embodiment of the present invention has been described and illustrated, various components useful therein, as well as variations in the apparatus itself, all coming within the spirit of this invention, will, no doubt, readily suggest themselves to those skilled in the art. For example, While the variable impedance device in the bias circuit of the electron gun has been described as a triode tube, it is within the contemplation of the present invention for this device to be, for example, a pentode tube or a semiconductor device of variable impedance. Hence, it is desired that the foregoing shall be considered as illustrative and not in a limiting sense.

What is claimed is:

1. Bias means for varying the intensity of current in an electron device having a cathode and a control electrode, said bias means comprising (a) a first impedance,

(b) a controllable impedance,

(0) means for connecting said cathode to said control electrode through at least a portion of said first impedance and through said controllable impedance in tandem,

((1) means for applying a first potential across at least a portion of said first impedance,

(e) means for applying a second potential across at least a portion of said controllable impedance, said first and second potentials being in series aiding relation with respect to said cathode and control electrode, and

(f) means to vary the impedance of said controllable impedance, whereby to vary the intensity of said beam.

2. Bias means for varying the intensity of current in an electron device having a cathode and a control electrode, said bias means comprising (a) a resistor,

(b) a variable impedance device having two main electrodes and a control electrode,

(c) means for connecting one of said main electrodes to said cathode through at least a portion of said resistor,

((1) means for connecting the other of said main electrodes to said control electrode of said electron device,

(e) means for applying a first potential across at least a portion of said resistor,

(f) means for applying a second potential across said main electrodes, said first and second potentials being in series aiding relation with respect to said cathode and the control electrode of said electron device, and

(g) means connected to said control electrode of said variable impedance device to vary the impedance thereof, whereby to vary the intensity of said beam.

3. Bias means for controlling the intensity of an electron beam from an electron gun having a cathode and a control electrode, said bias means comprising (a) a first impedance,

(b) a variable impedance device having two main electrodes and a control electrode,

(c) means for connecting said cathode through at least a portion of said first impedance to one of said main electrodes,

(d) means for connecting the other of said main elec trodes to said control electrode of said electron gun,

(e) means for applying a voltage which is positive on said cathode across at least a portion of said first impedance,

(f) a multivibrator comprising a pair of cross-connected electron devices,

(g) means for applying a voltage across one of said electron devices,

(h) means for applying a voltage across the other of said electron devices in series with the main electrodes of said variable impedance device, said voltage across said first impedance and that across said main electrodes of said variable impedance being in series aiding relation with respect to said cathode and said control electrode of said electron gun, and

(i) means for operating said multivibrator to apply different voltages to said control electrode of said variable impedance device to cause said variable impedance device alternately to exhibit high and low impedances, whereby the intensity of said beam is varied.

4. Bias means for controlling the intensity of an electron beam from an electron gun having a cathode and a control electrode, said bias means comprising (a) a resistor,

(b) a first electronic device having a common electrode,

a control electrode, and an output electrode,

(c) a connection from said cathode through at least a portion of said resistor to said output electrode,

(d) a connection between said common electrode and the control electrode of said electron gun,

(e) means for applying the proper potential across said resistor to bring about the operation of said electron (f) a pair of additional electronic devices each having a common electrode, a control electrode, and an output electrode.

(g) connections from the output electrode and the control electrode of one of said additional devices respectively to the control electrode and the output electrode of the other of said. additional devices, the common electrodes of said additional devices being connected together,

(h) means for applying a potential to the output electrode of one device of said pair of devices with respect to the common electrode thereof over a path including the output electrode and the common electrode of said first electronic device and a further resistor in tandem, said further resistor being connected between the common electrode of said first device and the output electrode of said one device of said pair of devices,

(i) a connection from the output electrode of said one device to the control electrode of said first device,

(j) means for applying an operating potential between the output electrode and the common electrode of the other of said pair of devices, and

(k) means for applying pulses of alternate polarity between the control electrode and the common electrode of one of said pair of devices whereby the intensity of said electron beam is varied.

References Cited by the Examiner UNITED STATES PATENTS 2,591,918 4/52 Cole 3l530 X 2,726,918 12/55 Hathaway 315-30 X 2,782,340 2/57 Siskel 328-267 X JOHN W. HUCKERT, Primary Examiner. ARTHUR GAUSS, Examiner. 

2. BIAS MEANS FOR VARYING THE INTENSITY OF CURRENT IN AN ELECTRON DEVICE HAVING A CATHODE AND A CONTROL ELECTRODE, SAID BIAS MEANS COMPRISING (A) A RESISTOR, (B) A VARIABLE IMPEDANCE DEVICE HAVING TWO MAIN ELECTRODES AND A CONTROL ELECTRODE, (C) MEANS FOR CONNECTING ONE OF SAID MAIN ELECTRODES TO SAID CATHODE THROUGH AT LEAST A PORTION OF SAID RESISTOR, (D) MEANS FOR CONNECTING THE OTHER OF SAID MAIN ELECTRODES TO SAID CONTROL ELECTRODE OF SAID ELECTRON DEVICE, (E) MEANS FOR APPLYING A FIRST POTENTIAL ACROSS AT LEAST A PORTION OF SAID RESISTOR, (F) MEANS FOR APPLYING A SECOND POTENTIAL ACROSS SAID MAIN ELECTRODES, SAID FIRST AND SECOND POTENTIALS BEING IN SERIES AIDING RELATION WITH RESPECT TO SAID CATHODE AND THE CONTROL ELECTRODE OF SAID ELECTRON DEVICE, AND 